Natural graphites are made up of layered planes of hexagonal arrays of networks of carbon atoms and typically exist in the shape of flakes in nature. These layered planes of hexagonally arranged carbon atoms are substantially flat and are oriented so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant layers of carbon atoms are joined together by weak Van der Waals forces. These natural graphites are soft and brittle and are typically difficult to form into a shape due to cracking along these layered planes. Such characteristics of graphites are well known to those skilled in the art, see, e.g., U.S. Pat. No. 5,149,518.
Natural graphites, however, may be formed into flexible sheets by compressing exfoliated graphite particles. Exfoliated graphite particles are formed by expanding the natural graphite flakes. In this expansion process, natural graphite flakes are intercalated by dispersing the flakes in a solution containing an oxidizing agent, for instance, a mixture of nitric and sulfuric acid. After the flakes are intercalated excess solution is drained from the flakes and after washing with water, the intercalated graphite flakes are dried. Upon exposure to high temperature, for instance 1,090-1,370° C. (2,000-2,500° F.), the particles of intercalated graphite expand in dimension as much as 80 to 1000 or more times their original volume in an accordion-like fashion in the direction perpendicular to the layered planes of the hexagonally arranged carbon atoms of the constituent graphite particles.
The exfoliated graphite particles are then compressed or compacted together, in the absence of any binder, so as to form a flexible integrated graphite sheet of desired thickness and density. The compression or compaction is carried out by passing a thick bed of expanded particles between pressure rolls or a system of multiple pressure rolls to compress the material in several stages into sheet material of desired thickness.
The sheet material formed from the exfoliated graphite particles, unlike the original graphite flakes, can be formed and cut into various shapes. The compression operation flattens the expanded graphite particles causing them to somewhat engage and interlock. The compression reorientates many of the carbon atoms from the perpendicular, accordion-like arrangement back into layered, parallel planes. Nevertheless, some carbon atoms remain in substantially nonparallel planes. These carbon atoms in the nonparallel planes increase the porosity of the sheet as compared to natural graphite having parallel planes of carbon atoms, provide engagement among parallel planes of carbon atoms to provide flexibility to the sheet, and allow mechanical shaping without substantial cracking. Furthermore, as the degree of compression increases, the degree of reorientation of carbon atoms from nonparallel planes into layered, parallel planes also increases, especially near the exterior surfaces of the sheet.
The density of the compressed exfoliated product is typically in the range of about 0.08 to 1.4 g/cc (5 to 90 lbs/ft3) which is lower than the density of natural graphite (or fully compressed graphite) having a bulk density of about 2.2 g/cc (140 lbs/ft3). As the density of a graphite product increases, the porosity of the graphite product typically decreases. Porosity, P, is defined as the fraction of the total volume of a porous substance that is occupied by the pores of the substance, as shown below in Formula I.P=VP/VT,  (I)where VP is the pore volume and VT is the total volume.
The pore volume, VP, of a porous material is the total volume, VT, less the volume of the fully compressed bulk material, V0, orVP=VT/V0  (II)The porosity of a porous substance may also be expressed in terms of densities, as shown below in Formula III.P=1−DPD0,  (III)where DP is the density of the porous material and D0 is the density of the fully compressed material.
From Formula III, the porosity of the compressed product is about 0.96 to about 0.36 for products having a density of about 0.08 and 1.4 g/cc, respectively. The porosity of a fully compressed material is zero because such a fully compressed material does not have pore volume. This above-calculated porosity is often referred to as true porosity because the volume of both open and sealed pore spaces are included. Apparent porosity is a measurement of just the open-pore space which is accessible to a fluid, such as nitrogen or mercury. The volume of such open pore space is then substituted for VP in Formula I.
A higher density or a lower porosity product is typically too stiff for use as flexible sheet graphite and is typically too mechanically weak to survive mechanical shaping processes. Some applications also require higher impermeability or greater mechanical strength than is typical for compressed graphite sheets. For instance, anode and cathode fluid-flow plates used in a fuel cell should be substantially impermeable to gaseous reactants and products, such as hydrogen and oxygen, to avoid undesired leakage of the reactants and products. For these applications, a sealant is impregnated into the graphite sheet to seal the porosity, thereby providing the sealed graphite plate with greater impermeability. A similar graphite plate without having its pores sealed with a sealant would not be sufficiently gas impermeable for use within a fuel cell.
The impregnation of a sealant into a graphite sheet is often difficult because the sealant may not readily penetrate into the internal porosity of the sheet due to layered, parallel planes of carbon atoms at the surface of the plate. These layered, parallel planes tend to block access of the internal porosity. Furthermore, as the density of the graphite sheet increases, e.g., from about 0.08 to about 1.4 g/cc (5 to 90 lbs/ft3), impregnation of a sealant into internal porosity becomes more difficult because the outer surface of the plate is characterized by a greater number of layered, parallel planes of carbon atoms. Impregnation of these sheets, especially the denser sheets, is often quite time-consuming, thus increasing the manufacturing costs associated with such plates.
One technique for impregnating a flexible graphite sheet is disclosed in U.S. Pat. No. 4,729,910. The disclosed technique is to de-aerate the sheet and to apply a sealant under conditions of reduced pressure to facilitate the movement of the sealant into the porosity of the sheet. The technique suffers the disadvantage of requiring expensive pressure reducing equipment and using a sealant dissolved in organic solvents to reduce the viscosity of the sealant to permit entry of the less viscous sealant mixture into the porosity of the sheet. The organic solvent is then removed under reduced pressure conditions before the sealant is heat cured. This technique thus requires multiple pressure reducing steps to seal a graphite sheet.
U.S. Pat. Nos. 5,885,728 and 5,902,762 discloses a technique where ceramic fibers are incorporated into the graphite sheets to facilitate subsequent sealant infusion. This technique, however, suffers from the disadvantage of introducing impurities, such as the ceramic fibers themselves, which can be harmful to certain applications, such as fuel cells where the surfaces of the graphite sheets are coated or proximal to precious metals, such as platinum.
Despite these efforts, a need exists for sealing flexible graphite sheets without introducing impurities potentially harmful to the final application and without requiring multiple, complex processing steps.